The term “ADD” on a glasses prescription stands for “Addition power,” which is a specific measurement required for correcting near vision. This value represents the extra magnification or focusing strength needed to see objects clearly at a close distance, such as when reading or using a smartphone. It is a necessary component for individuals who require separate corrections for both distance and near viewing.
Understanding the Need: The Role of Presbyopia
The requirement for Addition power is a direct consequence of presbyopia, a normal, age-related change in the eye. This condition develops as the crystalline lens inside the eye gradually loses its elasticity and becomes stiffer over time. In a younger eye, muscles surrounding the lens can easily change its shape to bring near objects into sharp focus, a process known as accommodation.
As the lens hardens, its ability to change shape diminishes, making it increasingly difficult to focus on anything within arm’s reach. People first notice this when they begin holding books, menus, or phones farther away to see the text clearly. Presbyopia typically becomes noticeable around the mid-40s and slowly progresses, necessitating an increase in the required ADD power over the years.
Defining the Addition Value
The Addition value is a precise measurement expressed in diopters (D), the unit used to define the optical power of a lens. On a prescription, the ADD value is always recorded as a positive number, usually ranging from +0.75 D to about +3.50 D. This positive value signifies the supplementary magnifying power added to the distance prescription.
The ADD power is typically the same for both the right and left eyes, as presbyopia is a systemic, symmetrical aging process affecting both eyes equally. It represents the total extra power needed to shift the focal point from distance to a comfortable near-working distance. The final near power of the lens is the sum of the distance prescription and the Addition power.
Clinical Methods for Determining the Addition
Determining the precise Addition power is a detailed process performed by an eye care professional; it cannot be reliably calculated at home. While age provides a general starting point (e.g., a person in their mid-40s might start at +1.00 D, progressing to higher values in their 60s), the final determination relies on functional, subjective testing. The eye doctor must establish the weakest positive lens power that allows for clear, comfortable vision at the patient’s customary reading distance.
One common method involves measuring the patient’s Amplitude of Accommodation (AA), the total remaining focusing power of the eye. A clinical rule suggests that the prescribed ADD should only utilize about half of the remaining AA to prevent eye strain during sustained near work. Another technique is the NRA/PRA balance test, which uses plus and minus lenses to find the widest range of clear vision at the near working distance.
The clinical process is fine-tuned by using near-point acuity cards to find the point where print becomes clear and then slightly blurred. This is often done with the patient looking through a device called a phoropter, or with trial lenses, while the doctor incrementally adjusts the power. The final decision is heavily weighted by the patient’s subjective preference and their specific vocational needs.
Translating ADD into Vision Correction
Once the eye care professional determines the Addition value, this number is incorporated into the physical design of the lens to create multifocal glasses. The ADD value defines the difference in power between the top (distance) portion and the bottom (near) portion of the lens. This power can be incorporated in a few different ways.
In a bifocal lens, the ADD power is placed in a distinct, visible segment at the bottom, creating two fixed focal points: one for distance and one for near. Trifocal lenses include an additional segment for intermediate vision, such as the distance to a computer screen, where the power is typically one-half of the full ADD value. Progressive lenses, which are the most common choice today, use the ADD power to create a seamless, gradual change in power from the distance zone to the full near zone, without harsh lines.